In conventional permanent magnet dc motors, field magnets are installed within the casing and a rotor is rotatably supported on a shaft surrounded by the magnets. Such electric motors are constructed such that coils are wound on the rotor for establishing magnetic fields, and therefore, a commutator is required for supplying electric current to the coils of the rotor. Electric power is applied to the coils through a brush in contact with segments of the commutator as the rotates. But, in such a type of electric motor, the disadvantage is seen that the contacting portions between the commutator and brush are damaged due to the frictions generated by the revolving motion of the commutator.
Therefore, the need for a contactless or brushless type electric motor has arisen. In the brushless type electric motors thus proposed, permanent magnets are used in the rotor, and the rotor is made to turn by repeatedly reversing the magnetic poles of the field magnetic cores. This type of motor is described in Korean Patent Application No. 86,2085. The invention discloses a multiphase bipolar brushless dc motor by providing that each phase has an independent driving circuit (electronic commutator), an independent field coil and an independent position sensor, in such a manner that the respective phases are parallelly connected to a common dc power source, and each sensor is arranged at the interval of a shaft angle ((2.pi./number of poles)/number of phases). The electrical position of the rotor is detected by sensors for the respective electrical phases to supply signals to the respective driving circuits to cause currents to flow through the coils of the respective phases. Thus, in order to make the motor efficiently revolve, the position of the sensor plate and the sensor is detected, and at least three or more phases are provided so that the electric angle (.pi./number of phases) will be determined by the number of the phases, and that the torque ripple will be improved accordingly as the number of the phase is increased.
However, in such an electric motor, the number of the field magnetic cores is increased, and at least three or more phases of electric current are required. Further, it has an independent driving circuit for each phase, and therefore. the construction of the motor is very complicated, as well as the starting being difficult.